This invention relates to an X-ray tube and more particularly an X-ray tube equipped with a control grid located between a cathode and an anode to control electrons emitted from the cathode.
Generally, a cathode of a control grid-equipped X-ray tube has a cathode body on which is mounted a focusing electrode means having first and second focusing electrodes and a filament. The control grid is disposed between the first and second focusing electrodes. The grid and second focusing electrode are electrically connected to the first focusing electrode, and the second focusing electrode has an opening toward which the filament extends and through which electrons emitted from the filament passes. The first and second focusing electrodes and control grid are made at the same potential level and upon application of a grid bias an electric current in the tube is controlled, thereby causing electrons passing through an opening of the second focusing electrode to impinge upon an anode target under a controlled fashion. As a result, a desired X-ray is obtained from a focus on the anode target. During the use of the tube, however, an ionization current of a residual gas in the tube flows into a grid circuit, tending to change the grid bias. When the bias voltage is changed, no same potential is obtained from the filament and first and second focusing electrodes and in consequence the focusing state of electrons passing through the opening of the second focusing electrode is varied. In this case, a focus pattern on the anode target is varied, making it impossible to positively obtain a desired X-ray. Furthermore, a bias voltage becomes positive due to the ionization current of the residual gas in the tube. In this case, it is impossible to provide a stable grid control and in the worst case the X-ray tube suffers damage.
To eliminate such drawbacks a control grid equipped X-ray tube is known in which the grid is insulated from the first focusing electrode, first and second electrodes are made at the same potential level with that of a filament, and a bias voltage is applied to the grid. Since the first focusing electrode is small, for example, 25 to 30 mm in diameter and 10 to 15 mm in thickness, however, a greater insulator can not be disposed between the grid and the first focusing electrode. When a high grid bias of -2000 to -3000V is applied it is impossible to entirely insulate the grid from the first focusing electrode and in consequence it is impossible to provide a stable grid bias.